Actually Nadal has also had some pretty good results with some mediocre partners.
I think Rafa has pretty good results, at least in the hard court doubles, because he has a better partner--Marc Lopez.
As we all saw in Miami (or was it IW?) earlier this year, when Roger and Stan played Rafa & Marc, Marc was the best player on court that evening, followed by Fed and Wawrinka. Rafa look a bit hopeless